Project Summary/Abstract Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique that shows promise as a therapeutic aid in treating neurological disorders, such as stroke and disorders of consciousness. However, significant debate exists as to its efficacy and mechanisms of action. To make progress, it is first necessary to determine where the currents flow in the brain and what the acute effects of these currents are on neural activity. Traditional, targeted current flow is performed by placing a large electrode directly on top of a targeted cortical region. More recently, tDCS electrode placement for targeted current flow has been assisted by sophisticated computer simulations. However, these computer simulations suffer from significant inaccuracies due to substantial differences across patients with highly varying lesioned brain anatomies. Empirically mapping where electric current flows in the brain is vital to verifying that tDCS is targeting brain region(s) of interest. Equally important is to verify that the targeted brain region, or networks related to it, have indeed altered their functional output in any measurable way. Yet, there are no neuroimaging techniques available that can measure neural effects during stimulation without generating artefactual signals. We propose to develop concurrent magnetic resonance imaging (MRI)/tDCS acquisition and processing techniques to fill this methodological gap. We will use 3D multi-gradient-recalled echo MRI to measure magnetic fields produced by tDCS and from them calculate the electric current flow. We will validate the measures with analytic predictions for phantoms and computational simulations for in vivo human recordings. Further, to accurately compare brain function during tDCS, we propose a concurrent tDCS/functional MRI acquisition and signal processing method that can measure blood-oxygen level dependent (BOLD) responses without stimulation artefacts. We will validate the approach on phantom models and in humans during rest and task performance. The goal of this validation work is to establish a functional MRI technique that can be trusted to measure the acute effects of tDCS. We anticipate that the results of the proposed experimental plan will provide invaluable tools towards a better understanding of the mechanisms of action of tDCS and in the future enable individualized targeted treatment. We will make these customized MRI acquisition protocols and analysis tools publicly available with a web-based platform for support among users.